feat: gate remote command execution behind allow_remote_command

CMD packets arrive as unauthenticated UDP datagrams, yet every hbc client
executed the shell command they carry without any opt-in. Add an
allow_remote_command config key, default false: when off, the command is
logged and refused with "Refused: allow_remote_command is false" (visible in
the server event log under the command service), and subprocess is never
reached. When on, the client warns at startup that it will execute CMD
packets.

Applied to all four clients that handle CMD — hbc, hbc_windows.py,
hbc_mini.py, and the C hbc_mini — since gating only one leaves the others
wide open. The C client reads the same key from ~/.hbc.json and needs a
rebuild to pick it up. UPD (self-update) is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

hbd/client/config.py

@@ -19,6 +19,9 @@ CLIENT_DEFAULTS = {
     # Host identity
     "owner": None,             # Optional username to set as this host's owner on the server
 
+    # Security
+    "allow_remote_command": False,  # Execute shell commands received in CMD packets from the server
+
     # Runtime flags
     "foreground": False,
     "verbose": False,